By Eric Jones
The 2026 growing season is beginning as temperatures begin to increase. The temperatures for March and April for 2026 are greater than the historical averages for Aberdeen (+1 °F), Brookings (+4 °F), Oacoma (+4 °F) and Pierre (+4 °F) (South Dakota State University Mesonet). The increased temperatures are favorable for growing conditions, however, great temperature shifts throughout the day (i.e., low of 32 °F and high of 75 °F) may adversely affect the activity of herbicide applied for preplant burndown.
Herbicides should be applied to actively growing weeds to be effective. While the temperatures are within 55 to 75 °F this spring, weeds are likely actively growing and can be sprayed with herbicides. However, when temperatures decrease to freezing or below freezing temperatures, the weeds become stressed and normal functions cease. When normal plant functions cease (i.e., weeds stop growing), the herbicide(s) may not be translocated throughout the plant which results in reduced effectiveness. If temperatures are below freezing, leaf tissue of the weeds can freeze and impede the translocation of the applied herbicide all together.
If greater temperature shifts are to occur, make the herbicide application as early as possible when temperatures exceed 55 °F. Spraying later in the day will result in poor activity because the weeds will experience the extreme temperature shift in less time. Tank mixing different herbicides together (example: glyphosate [Roundup] and 2,4-D) can often be more effective if decreased temperatures are realized after the application, compared with using a single herbicide. Use the appropriate adjuvants for each herbicide to be included in the application. The output for the application should be between 15 to 20 gallons per acre to ensure proper coverage so weeds can absorb as much herbicide as possible. While the preplant burndown application is important for managing early season weed interference, select herbicides that are effective for the weed species present in each field. Refer to the South Dakota Pest Management Guides for more information on herbicide effectiveness on various weed species.
Source : sdstate.edu